Extracellular ATP stimulates inositol phospholipid turnover and calcium influx in C6 glioma cells

Cross-Talk in Nucleotide Signaling in Glioma C6 Cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₄ and the ionotropic P2X₇ receptor in glioma C6 cells. P2Y₁ and P2Y₁₂ both respond to ADP, but while P2Y₁ links to PLC and elevates cytosolic Ca²⁺ concentration, P2Y₁₂ negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y₁ receptor strongly decreases and P2Y₁₂ becomes a major player responsible for ADP-evoked signal transduction. The P2Y₁₂ receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y₁ has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X₇ receptor, often responsible for apoptotic fate, is not involved in Ca²⁺elevation in C6 cells. The shift in nucleotide receptor expression from P2Y₁ to P2Y₁₂ during serum withdrawal, the cross talk between both receptors and the lack of P2X₇ activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

FGF-21, a novel metabolic regulator, has a robust neuroprotective role and is markedly elevated in neurons by mood stabilizers

Fibroblast growth factor-21 (FGF-21) is a new member of the FGF super-family and an important endogenous regulator of glucose and lipid metabolism. It has been proposed as a therapeutic target for diabetes and obesity. Its function in the central nervous system (CNS) remains unknown. Previous studies from our laboratory demonstrated that aging primary neurons are more vulnerable to glutamate-induced excitotoxicity, and that co-treatment with the mood stabilizers lithium and valproic acid (VPA) induces synergistic neuroprotective effects. This study sought to identify molecule(s) involved in these synergistic effects. We found that FGF-21 mRNA was selectively and markedly elevated by co-treatment with lithium and VPA in primary rat brain neurons. FGF-21 protein levels were also robustly increased in neuronal lysates and culture medium following lithium-VPA co-treatment. Combining glycogen synthase kinase-3 (GSK-3) inhibitors with VPA or histone deacetylase (HDAC) inhibitors with lithium synergistically increased FGF-21 mRNA levels, supporting that synergistic effects of lithium and VPA are mediated via GSK-3 and HDAC inhibition, respectively. Exogenous FGF-21 protein completely protected aging neurons from glutamate challenge. This neuroprotection was associated with enhanced Akt-1 activation and GSK-3 inhibition. Lithium-VPA co-treatment markedly prolonged lithium-induced Akt-1 activation and augmented GSK-3 inhibition. Akt-1 knockdown markedly decreased FGF-21 mRNA levels and reduced the neuroprotection induced by FGF-21 or lithium-VPA co-treatment. In addition, FGF-21 knockdown reduced lithium-VPA co-treatment-induced Akt-1 activation and neuroprotection against excitotoxicity. Together, our novel results suggest that FGF-21 is a key mediator of the effects of these mood stabilizers and a potential new therapeutic target for CNS disorders.

Cross-talk in nucleotide signaling in glioma C6 cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y(1), P2Y(2), P2Y(12), P2Y(14) and the ionotropic P2X(7) receptor in glioma C6 cells. P2Y(1) and P2Y(12) both respond to ADP, but while P2Y(1) links to PLC and elevates cytosolic Ca(2+) concentration, P2Y(12) negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y(1) receptor strongly decreases and P2Y(12) becomes a major player responsible for ADP-evoked signal transduction. The P2Y(12) receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y(1) has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X(7) receptor, often responsible for apoptotic fate, is not involved in Ca(2+)elevation in C6 cells. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) during serum withdrawal, the cross talk between both receptors and the lack of P2X(7) activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Neuronal and glial cell lines as model systems for studying P2Y receptor pharmacology

Investigation of the role of extracellular nucleotides in nervous system has been one of the main topics of the P2Y receptor research throughout the years. In parallel to numerous studies on primary culture systems, various neuronal and non-neuronal cell lines have been used to model in vitro the processes mediated by extracellular nucleotides. In this review article, a survey of expression profiles of G protein-coupled P2Y receptor subtypes in nervous-system-derived cell lines is presented, by analysing the receptor expression at the mRNA, protein, and functional level. The variability of receptor expression profiles in established cell lines is further discussed, bringing forward some general properties for neuronal and glial malignant cell lines.

Human melanomas express functional P2X7 receptors

Adenosine 5'-triphosphate is known to function as a potent extracellular messenger, producing its effects via a distinct family of cell surface receptors. Different receptor subtypes have been shown to modulate different cellular functions such as proliferation, differentiation and apoptosis. We have investigated the functional expression and apoptotic action of the P2 X (7) receptor in human malignant melanoma tissue and cells. Incubation of cells with the potent P2 X (7) receptor agonist 2'-3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate leads to a decrease in cell number, which is dose-dependent and reversible by the antagonist 1-N,O-bis-[5-isoquinoline-sulfonyl]-N-methyl-L-tyrosyl)-4-phenyl-piperazine. Synthesis of the P2 X(7) receptor by these cells has been established by reverse transcriptase-polymerase chain reaction, immunohistochemistry, immunocytochemistry and cellular accumulation of the fluorescent DNA-binding dye YO-PRO-1. The P2 X(7) receptors have been shown to mediate apoptotic actions of extracellular nucleotides and represent a novel target for melanoma therapy.

Arachidonic acid inhibits capacitative Ca2+ entry and activates non-capacitative Ca2+ entry in cultured astrocytes

Arachidonic acid (AA) plays important physiological or pathophysiological roles. Here, we show in cultured rat astrocytes that: (i) endothelin-1 or thapsigargin (Tg) induces store-depleted activated Ca(2+) entry (CCE), which is inhibited by 2-aminoethoxydiphenyl borane (2-APB) or La(3+); (ii) AA (10 microM) and other unsaturated fatty acids (8,11,14-eicosatrienoic acid and gamma-linoleic acid) have an initial inhibitory effect on the CCE, due to AA- or fatty acid-induced internal acid load; (iii) after full activation of CCE, AA induces a further Ca(2+) influx, which is not inhibited by 2-APB or La(3+), indicating that AA activates a second Ca(2+) entry pathway, which coexists with CCE; and (iv) Tg or AA activates two independent and co-existing non-selective cation channels and the Tg-induced currents are initially inhibited by addition of AA or weak acids. A possible pathophysiological effect of the AA-induced [Ca](i) overload is to cause delayed cell death in astrocytes.

P2Y purinergic receptors regulate the growth of human melanomas

Adenosine 5'-triphosphate is known to function as a potent extracellular messenger producing its effects via a distinct family of cell surface receptors. Different receptor subtypes have been shown to modulate different cellular functions such as proliferation, differentiation and apoptosis. We investigated the functional expression and proliferative action of metabotropic P2Y receptors in human melanoma tissue and cells. Expression of functional P2Y1, P2Y2 and P2Y6 receptor subtypes was established by reverse transcriptase polymerase chain reaction, immunohistochemistry and intracellular calcium measurements using a Fluorometric Imaging Plate Reader. Incubation of A375 melanoma cells with the P2Y1 receptor-selective agonist 2-methylthioadenosine-5-diphosphate caused a decrease in cell number which was dose-dependent, whereas incubation with the P2Y2 receptor agonist uridine triphosphate caused a dose-dependent increase in cell number. The action of extracellular nucleotides on P2Y receptors was shown to mediate the growth of melanomas and the P2Y1 receptor is a putative target for melanoma therapy.

Capacitative Ca2+ influx in glial cells is inhibited by glycolytic inhibitors

In non-excitable cells, stimulation of phosphoinositide (PI) turnover and inhibition of the endoplasmic reticulum (ER) Ca2+-ATPase are methods commonly used to deplete calcium stores, resulting in a capacitative Ca2+ influx (i.e., Ca2+ release-activated Ca2+ influx). Since this Ca2+ influx in glial cells has not been thoroughly investigated, we have used C6 glioma cells as a glial cell model to study this phenomenon. On adding cyclopiazonic acid (CPA) or thapsigargin (TG) (two ER Ca2+-ATPase inhibitors) in Ca2+-free medium, only a small transient increase in intracellular Ca2+ was seen. After depletion of the stored Ca2+, a marked Ca2+ influx, followed by a prolonged plateau, was seen on re-addition of extracellular Ca2+ ions (2 mM), i.e., capacitative Ca2+ influx. A similar effect was seen on adding ATP, known to deplete the inositol triphosphate (IP3)-sensitive Ca2+ store in C6 cells. After various degrees of store depletion, the amplitude of the capacitative Ca2+ influx was found to be highly dependent on the amount of Ca2+ remaining in the store. This Ca2+ influx was markedly inhibited by (1) La3+ and Ni2+, (2) SK&F 96365, econazole, and miconazole, and (3) membrane depolarization, clearly showing that this Ca2+ influx after store depletion in C6 cells is a capacitative mechanism. Interestingly, the capacitative Ca2+ influx can be inhibited by a reduction in intracellular ATP (ATPi) levels in glial cells. The role of ATPi in the capacitative Ca2+ influx is discussed.

Extracellular ATP increases intracellular calcium in cultured adult Schwann cells

We have previously reported that extracellular ATP causes a transient rise in intracellular calcium concentration ([Ca2+]i) in cultured Schwann cells derived from adult animals [Ansselin A. D. et al. (1994) Int. J. Neurosci. 74, 148]. In this study, the receptor mediating this response has been characterized. Established adult rat and rabbit Schwann cell cultures were loaded with fura-2 (acetoxymethyl ester, 10 micromol/l, 40 min, 37 degrees C). which indicated, by fluorescence imaging, a resting [Ca2+]i of 34.7 +/- 1.4 nmol/l (mean S.E., n=591). The cells were exposed to 100 micromol/l ATP, ADP, AMP, UTP and adenosine in defined medium for 1-2 min, and the change in [Ca2+]i was observed as a change in the Fura-2 ratio. Seventy-seven percent of adult rat Schwann cells (n=235) and 88% adult rabbit Schwann cells (n=356) responded to the presence of extracellular ATP (100 mmol/l) with a transient increase in [Ca2+]i (41 and 90 nmol/l from resting value, respectively), independent of the presence of [Ca2+]o. Calcium waves were observed in one experiment. The following order of agonist potency was observed: UTP= ATP>>ADP>AMP=adenosine. The agonists alpha,beta-methylene-ATP and 2-methylthio-ATP had a small effect on the cells, similar to AMP, and were mutually desensitizing. The ATP antagonist suramin blocked the response. We conclude that adult Schwann cells express a purinergic ATP receptor belonging to the G-protein-coupled P2u alpha subtype [O'Connor S. et al. (1991) Trends pharmac. Sci. 12, 137-141].

Mechanisms of intracellular calcium regulation in adult astrocytes

Microfluorimetric techniques were used to measure changes in intracellular calcium in astrocytes cultured from the forebrain of the adult rat. Application of ATP consistently raised intracellular calcium. The response persisted in the absence of extracellular calcium, but then quickly declined upon repeated agonist application. Thapsigargin abolished responses to nucleotides following depletion of the endoplasmic reticular calcium stores. Calcium release was inhibited by caffeine, but was dramatically increased through inositol phosphate receptor sensitization by the sulphydryl reagent thimerosal. Responses to repeated nucleotide applications resulted in a gradual decline of peak calcium concentrations, suggesting a (post)receptor-mediated desensitization or gradual depletion of the internal calcium stores. Subsequent application of ionomycin suggested intracellular calcium depletion as the relevant mechanism. Depletion of the internal calcium stores with ATP, ionomycin or thapsigargin failed to reveal a calcium influx pathway. These results suggest that the capacitative mechanism of calcium entry does not operate in response to nucleotide receptor activation in these cells, and that the immediate refilling of the internal calcium stores is primarily determined by re-uptake of cytosolic calcium into the endoplasmic reticulum. A complete refilling of this calcium store by extracellular calcium may be a much slower process. Control of these signal transduction pathways is crucial to the maintenance of the calcium/energy homeostasis of the adult astrocyte in the central nervous system.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.

Ca2+ x calmodulin prevents myristoylated alanine-rich kinase C substrate protein phosphorylation by protein kinase Cs in C6 rat glioma cells

Ionomycin stimulated membrane-associated protein kinase Cs (PKCs) activity in C6 rat glioma cells as much as the potent PKCs stimulator 12-O-tetradecanoyl phorbol 13-acetate (TPA). However, while TPA, as expected, powerfully stimulated the phosphorylation of the PKCs' 85-kDa myristoylated alanine-rich protein kinase C substrate (MARCKS) protein, ionomycin unexpectedly did not. Instead, ionomycin reduced the basal MARCKS phosphorylation. Pretreating the glioma cells with ionomycin prevented TPA-stimulated PKCs from phosphorylating the MARCKS protein. The stimulation of membrane PKCs activity and the prevention of MARCKS phosphorylation by ionomycin required external Ca2+ because they were both abolished by adding 5 mM EGTA to the culture medium. Recently (Chakravarthy, B. R., Isaacs, R. J., Morley, P., Durkin, J. P., and Whitfield, J. F. (1995) J. Biol. Chem. 270, 1362-1368), we proposed that Ca2+ x calmodulin complexes block MARCKS phosphorylation by the activated PKCs in keratinocytes stimulated by raising the external Ca2+ concentration. In the present experiments calmodulin prevented MARCKS phosphorylation by TPA-stimulated PKCs in glioma cell lysates, and this blockade was lifted by a calmodulin antagonist, the calmodulin-binding domain peptide. But, physiologically more significant, pretreating intact glioma cells with a cell-permeable calmodulin antagonist, calmidazolium, prevented ionomycin from blocking MARCKS phosphorylation by PKCs in unstimulated and TPA-stimulated cells. The effect of ionomycin on MARCKS phosphorylation was not due to the stimulation of Ca2+ x calmodulin-dependent phosphoprotein phosphatase, calcineurin, because cyclosporin A, a potent inhibitor of this phosphatase, did not stop ionomycin from preventing MARCKS phosphorylation. The ability of ionomycin to prevent TPA-stimulated PKCs from phosphorylating MARCKS depended on whether ionomycin was added before, with, or after TPA. Maximum blockade occurred when ionomycin was added before TPA but was less effective when added with or after TPA. These results indicate that Ca2+ x calmodulin can profoundly affect PKCs' signaling at the substrate level.

Effects of protein kinase A activation on endothelin- and ATP-induced signal transduction

C6 glioma cells possess endothelin ETA receptor and P2 purinoceptor coupled to two signaling pathways, i.e. phosphoinositide turnover and inhibition of adenylyl cyclase. In this study, the effects of raising cyclic AMP levels on the inositol phospholipid hydrolysis and adenylyl cyclase inhibition caused by endothelin-1 and ATP in C6 glioma cells were examined. Pretreatment with cAMP generating agents (forskolin, isoproterenol and cholera toxin) or dibutyryl cAMP for 10 min-3 h did not affect the inositol phosphate accumulation caused by endothelin and ATP. Long-term (8-24 h) pretreatment with isoproterenol, forskolin, cholera toxin or dibutyryl cAMP resulted in a 40-50% inhibition of endothelin- and ATP-stimulated inositol phosphate accumulation, whereas the EC50 values of endothelin and ATP were not affected. Consistent with the effects on endothelin and ATP, NaF-induced inositol phosphate formation was also inhibited by cAMP generating agents to a similar extent. Permeabilized cells from 24 h isoproterenol-or forskolin-pretreated C6 cells also showed a diminished Ca(2+)-sensitivity of phosphoinositide-specific phospholipase C and also attenuated the potentiation response caused by GTP gamma S. The inhibitory effects on adenylyl cyclase by endothelin, ATP and 2-methylthio-ATP were unaffected by 24 h pretreatment with isoproterenol or forskolin. Long-term treatment with dibutyryl cGMP did not affect the two signaling pathways caused by ATP and endothelin. It is concluded that the phosphoinositide turnover, but not the adenylyl cyclase inhibition caused by endothelin and ATP in C6 cells, was inhibited by protein kinase A-dependent pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C alpha, delta, epsilon and zeta in C6 glioma cells. TPA induces translocation and down-regulation of conventional and new PKC isoforms but not atypical PKC zeta

Both the cytosol and membrane in C6 glioma cells express abundance of PKC alpha, delta, zeta and trace amount of PKC epsilon by Western blot analysis with isozyme-specific antibodies. These characteristics make this cell line a good model to study the properties of different classes of PKC isoforms in one cell type. Exposure of the cells to 100 nM TPA for 10 min resulted in the translocation of conventional PKC alpha (cPKC alpha) and new PKC delta (nPKC delta) and -epsilon from the cytosolic to the membrane fraction, while left atypical PKC zeta (aPKC zeta) unaffected. The extent of translocation of cPKC alpha induced by TPA was more prominent than that of nPKC delta and nPKC epsilon. alpha-TPA, the inactive phorbol ester, did not induce translocation of these isozymes. After treatment of the cells with 1 microM TPA for 17 h, cPKC alpha, nPKC delta and nPKC epsilon were almost completely down-regulated, whereas aPKC zeta was still unaffected. The natural activators of this cell line, endothelin-1 and ATP also translocated cPKC alpha and nPKC delta. However, the extent of translocation induced by these two agonists was much less than that of TPA.